The nucleus is arguably one of the most important organelles in the cell, and yet surprisingly little is known about how its shape is maintained and what determines its size. The importance of nuclear shape is underscored by the fact that various diseases, including cancer and premature aging, are associated with changes in nuclear shape, and yet the relationship between nuclear shape and nuclear function is poorly understood. In most metazoan cells, the nuclear envelope (NE) undergoes cycles of assembly and disassembly in each and every cell cycle, and one of the key outstanding questions in the field is which proteins facilitate these processes. To gain insight into NE dynamics, we initiated the C. elegans nuclear architecture project. This project has two main components: (a) to examine in C. elegans the role of proteins identified in a yeast screen as being involved in nuclear architecture; and (b) to screen for additional gene/proteins involved in determining nuclear shape and size, and in affecting NE breakdown and reassembly. Following our observation in budding yeast that Cdc5, the yeast homolog of polo kinase, affects nuclear morphology, we examined the role of the C. elegans polo kinase, PLK-1, in NE dynamics. We discovered that PLK-1 is required for nuclear envelope breakdown during early embryogenesis. In particular, PLK-1 is essential for breaching the NEs that separate the maternal and paternal chromosomes. Following fertilization in C. elegans, the maternal and paternal chromosomes become encased in NEs. These pronuclei migrate towards each other and become juxtaposed such that between there two pronuclei there are 4 closely apposed membranes (2 from each NE). During mitosis, the NE disassembles and the nuclear membrane retreats into the rest of the ER. The disassembly of the NEs between the two pronuclei involves the formation of a membrane gap that spans all 4 membranes. The gap is critical for the mixing of the parental genomes. We found that when PLK-1 is down regulated, the gap does not form and the maternal and paternal genomes remain segregated in two separate nuclei in all subsequent embryonic divisions. We are currently investigating the mechanism by which this gap is formed. Specifically, we identified a number of genes that, when down-regulated, fail to form the aforementioned membrane gap and we are exploring their role in gap formation. Additionally, we are using focused ion beam/scanning electron microscopy (FIB/SEM) to understand the structure of the gap between the two pronuclei. Our analysis shows that prior to gap formation, the two pronuclei are linked via a number of membrane junctions, including a novel type of junction that we termed 3-way sheet junction. In the plk-1 mutant, these junctions do not form, suggesting that they are needed for membrane gap formation and, ultimately, parental genome mixing. Work is underway to uncover the mechanism by which the junctions between the two pronuclei form.